A 20-year veteran teacher is accused of assaulting a middle school student who refused to stand for the Pledge of Allegiance, the latest incident in which a protest over the flag has devolved into apparent violence.

The teacher is Karen Smith, who until recently taught physical education at Angevine Middle School, about 20 miles north of Denver. She couldn't immediately be reached for comment.

Officers were called to the school around noon Thursday, the Denver Post reported. Neither the school nor the police released details, and the student's name has not been released. No charges have been filed.

A school spokesman told the newspaper that the policy is to allow students to sit or stand during the pledge.

In a letter to families obtained by Denver CBS affiliate KCNC-TV, Principal Mike Medina told families that "we will have a substitute teacher working with some of our PE classes for the time being."

He added that Smith had been placed on paid administrative leave and that the school was "working closely with our partners at the Lafayette Police Department."

Protests involving the American flag have become a flash point in the last two years since then-NFL player Colin Kaepernick knelt during the national anthem to protest racial injustice.

His example was followed by other NFL players, athletes in other sports and countless people with similar stances.

Those who kneel see the act as a powerful, nonviolent way to protest racial injustice, particularly society's inaction when police kill unarmed African-Americans.

Critics say it is an unacceptable assault on the values and mores of the United States - and its most potent symbol. Others in this camp say the protests are a disrespectful slap in the face to veterans of all races who have fought or died for American freedoms.

The differences of opinion frequently boil over into the public sphere, debates that are always contentious and sometimes violent.

In October, a woman was recorded throwing her drink on two men who knelt during the national anthem at a Los Angeles Lakers game, according to the Daily Mail. "Excuse me, this is for the national anthem you pieces of s---," she yelled as she tossed the beverage. "Disrespect our flag and our country, and that's how we'll react."

In Oak Park and Evanston this week, high school student-athletes knelt to protest social injustice in demonstrations during the national anthem.

Some hope this is a teaching moment for America's youths.

Others called it an embarrassment to the country, underscoring the complexity of an intensifying...

The Long Island Diocese of Rockville Centre (New York), which runs a Catholic school system, said in September that students who kneel during the anthem would face "serious disciplinary action," according to the New York Times. Athletes who didn't "stand in a respectful manner" could see reduced playing time or be kicked off the team.

In October, a Native American high school football player successfully sued a California school district, which required students and staff to stand and remove their hats when the anthem was played, according to Yahoo Sports.

"Students like our client who conscientiously carry their values and ideals with them cannot be silenced or directed on what to say or not say by their school in this manner," Katie Traverso, an attorney for the student whose identity was not revealed in court documents, told the news organization.

Last year, President Donald Trump injected himself into the kneeling controversy.

In a September speech in Alabama, President Trump called for NFL owners to suspend or fire any "son of a b----" who doesn't stand for the national anthem.

The remarks touched on an exposed nerve in a league where most of the players are black and most of the fans are white.

As The Washington Post's Cindy Boren wrote in November, the NFL doesn't require its 1,600 players to stand. Owners, who met in October with players to discuss the issue, have declined to compel them to do so.

After Trump's statements, NFL players - and in some cases owners, coaches and staff - engaged in a collective show of solidarity, standing with arms locked or kneeling or remaining in the locker room during the anthem.